Ecology : Stone is the greenest building material

A report* produced by SISTech in collaboration with Heriot-Watt University, Edinburgh, for Historic Scotland comparing the embodied carbon of natural stone with other building materials quantifies the environmental advantages of using stone. Here Naeeda Crishna and Dr Suzy Goodsir, two of the authors of the report, and Annabel Cooper from SISTech, discuss the issues.

The UK’s ambitious carbon reduction targets compel everyone in the UK, from individuals to big business, to reduce their carbon emissions. But when the UK Committee on Climate Change estimates that buildings and the built environment account for 42% of total UK emissions, the pressure is really on for the construction industry to do its bit.

A relatively small proportion of these emissions can be attributed to the construction of buildings. Instead it is in the production of building materials like concrete, steel, bricks and stone where the significant carbon impact is found. This ‘embodied carbon’ refers to the emissions occurring from the extraction, processing and transport of building materials.

Although using natural stone bypasses the initial carbon impact associated with the production of materials such as steel and brick, the UK is a significant importer of cheaper, foreign natural stone from countries such as Spain, Italy, China, South Africa and Brazil. It is these imported stones that come at the greatest cost in terms of environmental impact.

Driven by ambitious carbon reduction targets in the Climate Change (Scotland) Act 2009, Historic Scotland commissioned sustainability researchers SISTech to understand the embodied carbon in natural stone used in the construction and repair of Scotland’s buildings.

As part of a bigger programme to tackle energy efficiency related issues in traditionally constructed buildings, the rationale behind the project was to understand the impact of imported stone compared with indigenously produced stone and to quantify the impact of the UK stone industry.

The main aims of the study were to quantify a carbon footprint of the sandstone, granite and slate produced in Scotland and the UK, and compare this with the footprint of imported stone.

SISTech, working with Heriot-Watt University, used a sample of Scottish and UK quarries to examine the carbon emissions at each stage in the extraction, processing and transport of the stone, aligned with the BSI PAS (Publicly Available Standard) 2050, the current UK standard for carbon accounting. This was the first time a study of this kind had been undertaken in this way.

An investigation into the structure, transport modes and stone producing areas within the biggest export countries was also carried out and used to model the carbon associated with transporting stone from Spain, Poland, India and China to Scotland.

The results of the research show that within the UK, the carbon footprints of sandstone and granite are lower than those of other building materials – 64kg of CO2 equivalent (CO2e) per tonne of sandstone and 93kg per tonne for granite, although the carbon embodied in UK slate is significantly higher (232kgCO2e per tonne) due largely to the amount of waste stone associated with the quarrying and processing of slate. The carbon footprints of stone and other common construction materials are shown in Figure 1.

Figure 2 shows the disaggregated footprints for each stone type and the allocation of carbon to the main stages in the life cycle of the stone.

As can be seen in Figure 2, the largest component of each stone footprint is attributable to the processing stage of the life cycle, mostly due to the different stages of processing (ie primary processing, secondary processing, finishing) and the variety of machinery associated with each stage. The largest source of emissions in the processing phase of the life cycle was found in the electricity used to run stone preparation machinery, dust extraction devices and water pumping machinery.

The main conclusion of this part of the research confirmed that indigenous natural stone is a low carbon building material compared with other construction materials.

The main carbon impacts associated with UK stone are related to processing the stone, transport of the stone to site and volume of waste produced.

However, quarrying and processing of sandstone and granite are not very energy intensive compared with the production processes of other materials, such as brick or concrete.

Provided the stone is locally or domestically sourced, the carbon emissions associated with it are less than other building materials. Slate, on the other hand, has a similar carbon footprint to brick and concrete.

Perhaps most notably, the research also concluded that there is potential for further reducing emissions in stone production by using electricity generated from renewable sources.

As the main source of carbon emissions at stone yards is electricity use, it would be possible to make a significant reduction in the carbon footprint of stone by using green electricity with low carbon intensity, further supporting the argument for British natural stone use in the construction industry.

In terms of imported stone, unsurprisingly the study team found that the carbon footprint of stone increases significantly with the distance it travels from the country of origin, as shown in Figure 3.

As expected, the impact is much larger when stone is sourced from India or China, resulting in an increase in the footprint of sandstone from China compared with UK stone of more than 550% .

Importing stone from Spain, however, only increases the embodied carbon content by 7% in the case of slate and 2% for granite, which is no more than transporting stone from one end of the UK to the other.

However, it is important to note that the estimates of embodied carbon of imported stone presented in the study are conservative as they assume the simplest, shortest scenarios and routes from source to use. One of the assumptions is that the stone is processed at the same place as it is quarried, which is not necessarily going to be true. The assumption is also made that it is transported directly to the UK, when in reality the transport logistics of global stone trade are far more tortuous.

The study team found that rough block is often exported from EU countries to China and Brazil for processing (taking advantage of lower labour costs) only for the processed stone to be brought back into the EU for distribution throughout Europe and export to other parts of the world. These results highlight the large carbon impact that can be associated with imported stone.

The study shows that locally sourced British stone is a low carbon alternative to high embodied carbon building materials such as brick and concrete and to stone imported from countries such as China and India.

But it also highlights that by using green electricity sources to power the processing of stone, the British stone industry could further reduce its carbon footprint and, with support from the British construction industry, the carbon footprint of the UK as a whole.

The message to the UK construction industry must be: now is the time to recognise the potential of green energy to power the indigenous stone industry and, in turn, to reduce the carbon emissions of the construction industry as a whole.

*The report is called Embodied Carbon in Natural Building Stone in Scotland. It was produced for Historic Scotland. Authors: Naeeda Crishna, Dr Suzy Goodsir, Professor Phil Banfill, Dr Keith Baker. It can be obtained free from SISTech ([email protected]  Tel: 0131 451 8162). It can also be downloaded from the website.